A Statistical Survey of Peculiar L and T Dwarfs in SDSS, 2MASS, and WISE

We present the final results from a targeted search for brown dwarfs with unusual near-infrared colors. From a positional cross-match of SDSS, 2MASS and WISE, we have identified 144 candidate peculiar L and T dwarfs. Spectroscopy confirms that 20 of the objects are peculiar or are candidate binaries. Nine of the 420 objects in our sample are young ($\lesssim$200 Myr; 2.1%) and another 8 (1.9%) are unusually red with no signatures of youth. With a spectroscopic $J-K_s$ color of 2.58 $\pm$ 0.11 mag, one of the new objects, the L6 dwarf 2MASS J03530419+0418193, is among the reddest field dwarfs currently known and is one of the reddest objects with no signatures of youth known to date. We have also discovered another potentially very low gravity object, the L1 dwarf 2MASS J00133470+1109403, and independently identified the young L7 dwarf 2MASS J00440332+0228112, first reported by Schneider and collaborators. Our results confirm that signatures of low gravity are no longer discernible in low to moderate resolution spectra of objects older than $\sim$200 Myr. The 1.9% of unusually red L dwarfs that do not show other signatures of youth could be slightly older, up to $\sim$400 Myr. In this case a red $J-K_s$ color may be more diagnostic of moderate youth than individual spectral features. However, its is also possible that these objects are relatively metal-rich, and so have an enhanced atmospheric dust content.Comment: 27 pages, 7 figures, 5 tables, accepted to A

The Prototypical Young L/T-Transition Dwarf HD 203030B Likely Has Planetary Mass

Upon its discovery in 2006, the young L7.5 companion to the solar analog HD 203030 was found to be unusual in being $\approx$200 K cooler than older late-L dwarfs. HD 203030B offered the first clear indication that the effective temperature at the L-to-T spectral type transition depends on surface gravity: now a well-known characteristic of low-gravity ultra-cool dwarfs. An initial age analysis of the G8V primary star indicated that the system was 130--400 Myr old, and so the companion between 12--31 $M_{\rm Jup}$. Using moderate resolution near-infrared spectra of HD 203030B, we now find features of very low gravity comparable to those of 10--150 Myr-old L7--L8 dwarfs. We also obtained more accurate near infrared and {\sl Spitzer}/IRAC photometry, and find a $(J-K)_{\rm MKO}$ color of $2.56\pm0.13$ mag---comparable to those observed in other young planetary-mass objects---and a luminosity of log($L_{\rm bol}/L_{\odot}$)$\,=\,-4.75\pm0.04$ dex. We further reassess the evidence for the young age of the host star, HD 203030, with a more comprehensive analysis of the photometry and updated stellar activity measurements and age calibrations. Summarizing the age diagnostics for both components of the binary, we adopt an age of 100 Myr for HD 203030B and an age range of 30--150 Myr. Using cloudy evolutionary models, the new companion age range and luminosity result in a mass of 11 $M_{\rm Jup}$ with a range of 8--15 $M_{\rm Jup}$, and an effective temperature of $1040\pm50$ K.Comment: 12 pages, 7 figures, accepted for publication in A

The Faintest WISE Debris Disks: Enhanced Methods for Detection and Verification

In an earlier study, we reported nearly 100 previously unknown dusty debris disks around Hipparcos main-sequence stars within 75 pc by selecting stars with excesses in individual WISE colors. Here, we further scrutinize the Hipparcos 75 pc sample to (1) gain sensitivity to previously undetected, fainter mid-IR excesses and (2) remove spurious excesses contaminated by previously unidentified blended sources. We improve on our previous method by adopting a more accurate measure of the confidence threshold for excess detection and by adding an optimally weighted color average that incorporates all shorter-wavelength WISE photometry, rather than using only individual WISE colors. The latter is equivalent to spectral energy distribution fitting, but only over WISE bandpasses. In addition, we leverage the higher-resolution WISE images available through the unWISE.me image service to identify contaminated WISE excesses based on photocenter offsets among the W3- and W4-band images. Altogether, we identify 19 previously unreported candidate debris disks. Combined with the results from our earlier study, we have found a total of 107 new debris disks around 75 pc Hipparcos main-sequence stars using precisely calibrated WISE photometry. This expands the 75 pc debris disk sample by 22% around Hipparcos main-sequence stars and by 20% overall (including non-main-sequence and non-Hipparcos stars)

ADAPTIVE OPTICS OBSERVATIONS OF VEGA: EIGHT DETECTED SOURCES AND UPPER LIMITS TO PLANETARY-MASS COMPANIONS

ABSTRACT From adaptive optics observations with the Palomar 5 m telescope we place upper limits on the masses of any planetary companions located between $30 and 230 AU away from Vega, where our data are sensitive to depths ranging from H ¼ 12:5 19:0 mag fainter than Vega itself. Our observations cover a plus-shaped area with two 25 00 Â 57 00 elements, excluding 7 00 Â 7 00 centered on the star. We have identified two double and four single point sources. These projected companions are 14.9-18.9 mag fainter than Vega and, if physically associated, would have masses ranging from 4 to 35 M J and orbital radii of 170-260 AU. Recent simulations of dusty rings around Vega predict the presence of a perturbing body with a mass of less than 2-3 M J and an orbital radius of$40-100 AU, and more massive (d10 M J ) planets cannot be excluded. None of the detected objects are this predicted planet. Based on a color-magnitude, spectroscopic, and proper motion analysis, all objects are consistent with being background sources. Given the glare of Vega, a 2 M J object near the expected orbital radii would not have been visible at the 5 level in our data, though any brown dwarf with mass greater than 10 M J could have been seen at a separation greater than 80 AU

Adaptive Optics Observations of Vega: Eight Detected Sources and Upper Limits to Planetary‐Mass Companions

From adaptive optics observations with the Palomar 5 m telescope we place upper limits on the masses of any planetary companions located between ~30 and 230 AU away from Vega, where our data are sensitive to depths ranging from H = 12.5-19.0 mag fainter than Vega itself. Our observations cover a plus-shaped area with two 25″ × 57″ elements, excluding 7″ × 7″ centered on the star. We have identified two double and four single point sources. These projected companions are 14.9-18.9 mag fainter than Vega and, if physically associated, would have masses ranging from 4 to 35 M_J and orbital radii of 170-260 AU. Recent simulations of dusty rings around Vega predict the presence of a perturbing body with a mass of less than 2-3 M_J and an orbital radius of ~40-100 AU, and more massive (≾10 M_J) planets cannot be excluded. None of the detected objects are this predicted planet. Based on a color-magnitude, spectroscopic, and proper motion analysis, all objects are consistent with being background sources. Given the glare of Vega, a 2 M_J object near the expected orbital radii would not have been visible at the 5 σ level in our data, though any brown dwarf with mass greater than 10 M_J could have been seen at a separation greater than 80 AU

A Substellar Common Proper Motion Companion to the Pleiad HII 1348

We announce the identification of a proper motion companion to the star HII 1348, a K5V member of the Pleiades open cluster. The existence of a faint point source 1.1arcsec away from HII 1348 was previously known from adaptive optics imaging by Bouvier et al. However, because of a high likelihood of background star contamination and in the absence of follow-up astrometry, Bouvier et al. tentatively concluded that the candidate companion was not physically associated with HII 1348. We establish the proper motion association of the pair from adaptive optics imaging with the Palomar 5m telescope. Adaptive optics spectroscopy with the integral field spectrograph OSIRIS on the Keck 10m telescope reveals that the companion has a spectral type of M8\pm1. According to substellar evolution models, the M8 spectral type resides within the substellar mass regime at the age of the Pleiades. The primary itself is a known double-lined spectroscopic binary, which makes the resolved companion, HII 1348B, the least massive and widest component of this hierarchical triple system and the first substellar companion to a stellar primary in the Pleiades.Comment: accepted by Ap

Multi-wavelength modeling of the spatially resolved debris disk of HD 107146

(abridged) We aim to constrain the location, composition, and dynamical state of planetesimal populations and dust around the young, sun-like (G2V) star HD 107146}. We consider coronagraphic observations obtained with the Advanced Camera for Surveys (HST/ACS) onboard the HST in broad V and broad I filters, a resolved 1.3mm map obtained with the Combined Array for Research in Millimeter-Wave Astronomy (CARMA), Spitzer/IRS low resolution spectra, and the spectral energy distribution (SED) of the object at wavelengths ranging from 3.5micron to 3.1mm. We complement these data with new coronagraphic high resolution observations of the debris disk using the Near Infrared Camera and Multi-Object Spectrometer (HST/NICMOS) aboard the HST in the F110W filter. The SED and images of the disk in scattered light as well as in thermal reemission are combined in our modeling using a parameterized model for the disk density distribution and optical properties of the dust. A detailed analytical model of the debris disk around HD 107146 is presented that allows us to reproduce the almost entire set of spatially resolved and unresolved multi-wavelength observations. Considering the variety of complementary observational data, we are able to break the degeneracies produced by modeling SED data alone. We find the disk to be an extended ring with a peak surface density at 131AU. Furthermore, we find evidence for an additional, inner disk probably composed of small grains released at the inner edge of the outer disk and moving inwards due to Poynting-Robertson drag. A birth ring scenario (i.e., a more or less broad ring of planetesimals creating the dust disk trough collisions) is found to be the most likely explanation of the ringlike shape of the disk.Comment: 15 pages, 9 figures, accepted for publication in A&

Weather on Other Worlds. VI. Optical Spectrophotometry of Luhman 16B Reveals Large-amplitude Variations in the Alkali Lines

Using a novel wide-slit, multiobject approach with the GMOS spectrograph on the 8 m Gemini South telescope, we have obtained precise time-series spectrophotometry of the binary brown dwarf Luhman 16 at optical wavelengths over two full nights. The B component of this binary system is known to be variable in the red optical and near-infrared with a period of 5 hr and an amplitude of 5%-20%. Our observations probe its spectrally resolved variability in the 6000-10000 Å range. At wavelengths affected by the extremely strong, broadened spectral lines of the neutral alkali metals (the potassium doublet centered near 7682 Å and the sodium doublet at 5893 Å), we see photometric variations that differ strikingly from those of the 8000-10000 Å "red continuum" that dominates our detected flux. On UT 2014 February 24, these variations are anticorrelated with the red continuum, while on February 25 they have a large relative phase shift. The extent to which the wavelength-dependent photometric behavior diverges from that of the red continuum appears to correlate with the strength of the alkali absorption. We consider but ultimately reject models in which our observations are explained by lightning or auroral activity. A more likely cause is cloud-correlated, altitude-dependent variations in the gas-phase abundances of sodium and potassium, which are in chemical equilibrium with their chlorides in brown dwarf atmospheres. Clouds could influence these chemical equilibria by changing the atmospheric temperature profile and/or through cloud particles acting as chemical catalysts